Phytophthora Root Rot

Phytophthora root rot causes a slow decline of the tree, especially in new plantings. The leaves turn light green or yellow and may drop, depending on the amount of infection. The disease destroys the feeder roots of susceptible rootstocks. The pathogen infects the root cortex, which turns soft and separates from the stele. If the destruction of feeder roots occurs faster than their regeneration, the uptake of water and nutrients will be severely limited. The tree will grow poorly, stored energy reserves will be depleted, and production will decline.

Disease symptoms are often difficult to distinguish from nematode, salt, or flooding damage; only a laboratory analysis can provide positive identification.

Phytophthora species are present in most citrus groves. They can survive adverse conditions as persistent spores in the soil. During moist conditions, large numbers of motile zoospores, which can swim in water for short distances, are produced. Zoospores are the infective agents that are carried in irrigation or rainwater to the roots.

Phytophthora citrophthora is a winter and summer root rot that also causes fruit brown rot and gummosis. Phytophthora citrophthora is most damaging when citrus roots are inactive and their resistance to infection is low.

Phytophthora parasitica is active during warm weather when roots are growing.

Management of Phytophthora root rot involves the use of resistant rootstocks, irrigation management, fungicides, and fumigation.

Provide adequate soil drainage and avoid over irrigation. If destruction of feeder roots is minimal, corrective action may include increasing irrigation intervals, switching to alternate middle row irrigation or a different irrigation system such as mini sprinklers, and installing subsoil tiles.

When replanting or establishing new plantings, choose resistant rootstocks where possible, but also consider tolerance to other diseases, nematodes, and cold. The most tolerant rootstocks are trifoliate orange, swingle citrumelo, citrange, Alemow, and sour orange.

Use cultural controls and resistant rootstocks in an organically managed citrus grove.

If a tree growing on susceptible rootstock looks stressed, dig up some soil and check the feeder roots. Sample for P. parasitica during July through September, and P. citrophthora throughout the year:

• Randomly select 20 to 40 locations within a 10-acre orchard block with mild to moderate expected Phytophthora tree decline.
• Sample from aroung the tree drip line or near irrigation emitter where roots are concentrated.
• Put composite samples in a sealed plastic bag, but do not refrigerate or overheat.
• Ship within 24 to 48 hours to a lab where propagule count per unit of soil and root infestation are determined.
• Phytophthora populations of more than 15 to 20 propagules per gram of root zone soil may warrant a pesticide application. When planting or replanting in soil infested with Phytophthora, or when a susceptible rootstock has to be used, fumigation may be feasible if no other adverse conditions persist.